openwrt uart setup USB TTL converter

I wanted to change the bootloader on my openwrt board to boot from msata or micro sd card. I'm getting a lot more familiar with this tiny foot print SOC Linux. I have a few projects I'm working on.
TTL Converter in this video
https://www.amazon.com/gp/product/B072K3Z3TL/ref=as_li_tl?ie=UTF8&tag=fintechcomm0f-20&camp=1789&creative=9325&linkCode=as2&creativeAS=B072K3Z3TL&linkId=16bafeed2fcefdd264dcccba1e37bf95

My Favorite Board U7621-06
http://www.unielecinc.com/q/news/cn/p/product/detail.html?qd_guid=pyrEjfTmYf

My Second Favorite Board U7621-01
http://www.unielecinc.com/q/news/cn/p/product/detail.html?qd_guid=MDSixFKqTp

Older Board U7628-01
https://www.aliexpress.com/item/32816981605.html?spm=a2g0o.productlist.0.0.6ebe61242P7R19&algo_pvid=dd040caf-7323-427d-8da1-ac302041abe6&algo_exp_id=dd040caf-7323-427d-8da1-ac302041abe6-25&pdp_ext_f=%7B%22sku_id%22%3A%2264789943590%22%7D&pdp_pi=-1%3B39.69%3B-1%3B-1%40salePrice%3BUSD%3Bsearch-mainSearch